The invention relates to a method for the transmission of data packets in a mobile radio system, and a corresponding mobile radio system.
In mobile radio systems such as GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunications System) data and data packets are transmitted from the fixed network by transmission stations to receiver stations by way of an air interface. FIG. 1 shows a known UMTS in schematic and simplified form. The fixed network CN is connected by data lines Iu to the UTRAN (Universal Terrestrial Radio Access Network). The UTRAN comprises a plurality of subsystems RNS (Radio Network Subsystem) which each have a data line Iu to the fixed network CN. Always connected to each data line Iu is a controller RNC (Radio Network Controller) which is in turn connected by connections Iub to a plurality of base stations B. A plurality of cells of the mobile radio system can be assigned to each base station B (e.g. by way of sector antennas). The interface between UTRAN and the user equipment UE which is referred to in GSM as a mobile station is implemented by way of the interface Uu by a radio link. The controllers RNC of different subsystems RNS are as a rule connected by a further interface Iur. This further interface Iur is required for a so-called handover (switch of cell), amongst other things.
During a soft handover, the user equipment UE is simultaneously connected to at least two base stations B which are each capable of being assigned to different controllers RNC. Since only one connection Iu is required in order to exchange data between user equipment UE and fixed network CN, the data flow is controlled by only one controller RNC, called SRNC (Serving RNC). If the two base stations B are to be assigned to different controllers RNC, only one of the controllers RNC is the SRNC, while the second controller is referred to as the DRNC (Drift RNC). The controller known as DRNC handles its data transmission to and from the fixed network CN by way of the further interface Iur and thus by way of the controller SRNC.
In each controller RNC, a connection check RLC (Radio Link Control) is performed with the aid of a storage facility RS in which, apart from the data packets, information is stored concerning which data has been sent, is to be sent again, or is yet to be sent. In the case of a soft handover, this data transmission status information is however stored only in the controller SRNC. The user equipment UE also has a similar storage facility US for status information and data packets, which stores information about which data it has successfully decoded and for which data it is requesting a re-transmission. In the event of successful decoding of the data, the user equipment UE sends a confirmation ACK (acknowledge) to the radio link control RLC facility for the relevant controller RNC. If the decoding is not successful, a NACK (non acknowledge) is sent instead.
Whereas during the soft handover a connection exists simultaneously between the user equipment UE and at least two base stations B, during the hard handover the connection is only transferred from one base station B to another base station B if the connection to the first base station B has been previously terminated. Just as in the case of the soft handover, this connection handover can take place both between base stations B of one and the same controller RNC and also between base stations B of different controllers RNC. Following a hard handover, however, a synchronization of the stored data transmission status information must take place between the user equipment UE and the relevant controller RNC and, where necessary, the storage contents of the old controller RNC must be transmitted to the new controller RNC. This synchronization of the data transmission status information takes time and delays the resumption of data transmission following a hard handover. High data transfer rates cannot therefore be achieved in this manner.
The same holds true in respect of the data transfer rate in the case of a fast selection of the radio cell with which the user equipment UE wishes to operate a connection. The user equipment UE has a set of radio cells for possible selection by way of which it can route the connection to the fixed network CN. The user equipment UE then determines the radio cell having the best properties and signals in the uplink direction the cell from which it wishes to be supplied. This principle is referred to as Fast Cell Selection (FCS). If the chosen cell changes during a connection, then the same problem occurs with FCS as in the case of a hard handover. The synchronization of the stored data transmission status information must also take place for FCS as described in the previous paragraph. High data transfer rates are not therefore possible in this manner even in the case of FCS.
Future mobile communication systems will however require, and need to support, high data transfer rates. One example of this is High Speed Downlink Packet Access (HSDPA) which is currently under discussion by the 3rd Generation Partnership Project (3GPP) for UTRA FDD and TDD (Universal Terrestrial Radio Access Frequency Division Duplex and Time Division Duplex). In order to achieve high data transfer rates, control of the data transmission is shifted from the controller RNC into the base stations B, in other words in the base stations B additional storage facilities BS are set up which store the data packets and the data transmission status information. Time savings are achieved in this way since the transmission path between the controller RNC and the base stations B is dispensed with during control of the data transmission. Even when this new storage facility BS is included in the base stations B the hard handover takes place as described previously. Proposals on this topic may be found for example in a paper presented by Motorola on the occasion of the TSG-RAN Working Group 2 meeting #18/00 in Edinburgh, 15 to 19 Jan. 2000, under the title “Fast Cell Selection and Handovers in HSDPA” (R2-A010017). As explained above, the data transmission is resumed when the new base station B has been informed of the status of the data transmission, in other words when the storage contents of the old base station B have been sent to the new base station B. This synchronization of the data transmission status information between old and new base station B takes place in this situation either by way of the connections Iub on the controller RNC and if applicable also by way of the further interface Iur and/or by the user equipment UE by way of the radio interface. The limits for HSDPA are thus given as a result of the finite time which is required for transmission of the storage contents of the old base station B (data packets and transmission status of the data packets) to the storage facility BS of the new base station B.
A handover from an old base station to a new base station for a cellular computer device is described in EP0695053A. The protocol status for the data transmission is notified to the new base station either by appropriate information from the cellular computer device, information from the old base station or a combination of information from both sources during the handover.
A method for operating a mobile radio network is described in DE 100 17 062 A1, whereby in the case of a switch of connection for a mobile station from a first base station to a second base station transmission-specific information is transmitted from a first higher-level network unit to a second higher-level network unit in order to resume a transmission of data units to the mobile station after the switch in connection from the current status.